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SLOPE FAILURE ASSESSMENT

IN PENANG ISLAND

USING GEOELECTRICAL METHODS

by

MUHAMAD IQBAL MUBARAK

FAHARUL AZMAN

Thesis submitted in fulfilment of the requirements

for the degree of

Master in Science

April 2018

ii

ACKNOWLEDGEMENT

I thank Allah S.W.T for His mercy for giving me this opportunity to further

study in Master’s degree and His guidance for me to face challenges in order to

complete this research.

I would like to express my sincere gratitude to my main supervisor, Dr Nur

Azwin Ismail for her continuous support, guidance and encouragement that leads me

to completing my research. A special thanks to my co – supervisor, Dr Nordiana Mohd

Muztaza for her invaluable advices and suggestions. Many thanks also to all

Geophysics laboratory assistants for their time and effort in assisting me to conduct

my research.

My appreciation also to all my very helpful and supportive postgraduate

students for their non – stop encouragement and knowledge sharing that allow me to

move forward.

My sincere appreciation toward my beloved parents Mr. Faharul Azman

Ahmad Sabki and Mrs. Faridah Md Saad and also my siblings for their prayers and

encouragements. Last but not least, Fellowship Scheme of Universiti Sains Malaysia

for sponsoring my tuition fees and allowances during my study period.

iii

TABLE OF CONTENTS

Acknowledgement ii

Table of Contents iii

List of Tables vii

List of Figures x

List of Abbreviations xiv

List of Symbols xvi

Abstrak xviii

Abstract xx

CHAPTER 1: INTRODUCTION

1.0 Preface 1

1.1 Problem statements 3

1.2 Research objectives 5

1.3 Scope of research 5

1.4 Significant of study 6

1.5 Thesis outlines 7

CHAPTER 2: LITERATURE REVIEW

2.0 Introduction 8

2.1 Previous studies 8

2.2 Basic theory of 2D resistivity method 25

2.3 Basic theory of induced polarization 28

2.4 Depth of penetration 30

iv

2.5 Porosity 30

2.6 Moisture content 31

2.7 Bulk density 31

2.8 Summary 32

CHAPTER 3: MATERIALS AND METHODS

3.0 Introduction 33

3.1 Geology and geomorphology of Penang Island 34

3.2 Study area 37

3.2.1 Sungai Batu 42

3.2.2 Bukit Relau 43

3.2.3 Air Hitam 44

3.3 Laboratory test 45

3.3.1 Moisture content measurement 46

3.3.2 Porosity calculation 46

3.3.3 Particle size distribution (PSD) analysis 47

3.3.3(a) Experiment procedure 48

3.3.3(b) Coefficient of gradation, Cc and coefficient

of uniformity, Cu

48

3.3.3(c) Particle size statistics of mean and sorting 50

3.4 Geophysical methods 51

3.4.1 2D resistivity method 52

3.4.2 Induced polarization (IP) method 54

3.4.3 Electrical properties of materials 54

3.4.4 Data acquisition 55

v

3.4.5 Data processing 57

3.5 Monitoring the study area 58

3.6 Rainfall distribution 59

3.7 Regression analysis 60

3.8 Summary 61

CHAPTER 4: RESULTS AND DISCUSSION

4.0 Introduction 63

4.1 Results 63

4.1.1 Sungai Batu 64

4.1.1(a) Laboratory test 64

4.1.1(b) Particle size distribution (PSD) analysis

65

4.1.1(c) Rainfall distribution 67

4.1.1(d) Geophysical methods 69

4.1.1(e) Slope monitoring 71

4.1.1(f) Changes within Sungai Batu study area

subsurface

75

4.1.2 Bukit Relau 81


4.1.2(b) Particle size distribution (PSD) analysis

82

4.1.2(c) Rainfall distribution 84

4.1.2(d) Geophysical methods 86

4.1.2(e) Slope monitoring 88

4.1.2(f) Changes within Bukit Relau study area

subsurface

91

4.1.3 Air Hitam 93

vi


4.1.3(b) Geophysical methods 94

4.2 Empirical correlation between soil moisture content and porosity

96

4.3 Empirical correlation between geophysical data and laboratory

tests

97

4.4 Empirical correlation between geophysical data and rainfall

distribution

99

4.5 Summary 101

CHAPTER 5: CONCLUSION AND RECOMMENDATIONS

5.0 Conclusion 105

5.1 Recommendations for future work 109

REFERENCES 110

APPENDICES

vii

LIST OF TABLES

Page

Table 1.1 Series of major slope failure occurrences in Malaysia and

consequences in terms of deaths from 2002 – 2017.

1

Table 1.2 Summary of slope failures in Penang Island.

4

Table 2.1 Summary of resistivity value with interpretation at the study

areas in Selangor (Muztaza et al., 2017).

17

Table 2.2 Resistivity range of weathered granite and fresh granite in

Malaysia (Yaccup and Tawnie, 2015).

18

Table 2.3 Saturated peak shear box and particle size distribution tests

(Hashim et al., 2015).

24

Table 2.4 Result of soil classification and particle size distribution test


24

Table 2.5 Porosity of soil according to the percentage of porosity (Faur

and Szabo, 2011).

31

Table 2.6 Relationship of soil bulk density for root growth based on

soil texture (Arshad et al., 1996).

31

Table 3.1 Description of weathering grade (Attewell, 1993).

41

Table 3.2 Name of equipment as labelled in Figure 3.11.

45


48

Table 3.4 Summary of soil classification (Santamarina et al., 2001).

49

Table 3.5 Conversion from millimetre to phi unit (Folk and Ward,

1957).

50

Table 3.6 Values of sorting and the interpretation (Folk and Ward,

1957).

51

Table 3.7 Resistivity of various rock and sediment (Telford, 1990).

55

Table 3.8 Chargeability of various material (Telford, 1990).

52


57

Table 3.10 Categories of precipitation and the description (Department

of Meteorological Malaysia, 2018).

59

viii

Table 3.11 Strength of correlation (Evan, 1996).

60

Table 4.1 Laboratory test for Sungai Batu study area.

64

Table 4.2 Particle size distribution analysis result at Sungai Batu study

area.

65

Table 4.3 Mean and standard deviation of Sungai Batu soil samples.

66

Table 4.4 Summary of resistivity value with interpretation and feature

found at Sungai Batu study area.

69

Table 4.5

Summary of chargeability value with interpretation at Sungai

Batu study area.

70

Table 4.6 Laboratory test for Bukit Relau study area.

82

Table 4.7 Particle size distribution analysis result at Bukit Relau study

area.

83

Table 4.8 Mean and standard deviation of Bukit Relau soil samples.

83

Table 4.9 Summary of resistivity value with interpretation and features

found at Bukit Relau study area.

86

Table 4.10 Summary of chargeability value with interpretation and

features found at Bukit Relau study area.

87

Table 4.11 Laboratory test for Air Hitam study area.

94

Table 4.12 Summary of resistivity value with interpretation and features

found at Air Hitam study area.

94

Table 4.13 Summary of chargeability value with interpretation and

features found at Air Hitam study area.

95

Table 4.14 Categories of resistivity value obtained in this research.

101

Table 4.15 Categories of chargeability value obtained in this research.

101

Table 4.16 Summary of result obtained from Sungai Batu study area.

102

Table 4.17 Summary of result obtained from Bukit Relau study area.

103

Table 4.18 Summary of result obtained from Air Hitam study area.

103

Table 4.19 Summary of empirical correlation between geophysical data,

laboratory tests and rainfall distribution.

104

ix

Table 5.1 Summary of slope failure potentials and the factor for failure

at the study areas.

108

x

LIST OF FIGURES

Page

Figure 2.1 ERT profiles through Buzad landslide in (a) 2007 (b) 2012

(c) 2014 (Popescu et al., 2014).

10

Figure 2.2 Resistivity profiles for ERT1 – ERT3 (Giocoli et al., 2015).

13

Figure 2.3 3D fence diagram for resistivity section in Aydin (Drahor et

al., 2006).

14

Figure 2.4 Resistivity profiles according to the location (a) top of

landslide (b) toe of landslide (Hazreek et al., 2017).

18

Figure 2.5 a) Average rainfall at Cameron Highland from March 2008

to Dec 2008 b) Observed total displacement at Prisms I-VI

(Khan et al., 2010).

20

Figure 2.6 Safety factor caused by combination of rainfall and the

water level fluctuation (Liu and Li, 2015).

22

Figure 2.7 The flow of current from a point current source (current

electrode) and the resulting potential distribution within the

subsurface (Loke, 2001).

28

Figure 2.8 Equilibrium ion distribution (left). Polarization in the

electric field direction (right) (Slater and Lesmes, 2002).

29

Figure 2.9 Principle of time domain IP signal (a) current on – off (b)

the measured voltage decay (modified from Slater and

Lesmes, 2002).

30

Figure 3.1 Location of study areas (Google Earth, 2018).

35

Figure 3.2 General geology of Penang Island (Ong, 1993).

36

Figure 3.3 Lineament map of Penang Island (Ong, 1993).

37

Figure 3.4 Location of study areas on topography map.

38

Figure 3.5 Slope in degree for cell 10 units of Penang Island with

study area locations (Azmi, 2014).

39

Figure 3.6 Rainfall distribution from 2014 to 2016 (Department of

Meteorological Malaysia, 2018).

40

Figure 3.7 Number of days with daily amount of rainfall more or equal

to 20 mm/day in 2014 – 2016.

41

xi

Figure 3.8 Aerial view of Sungai Batu survey line (Google Earth,

2018).

43

Figure 3.9 Aerial view of Bukit Relau survey line (Google Earth,

2018).

44

Figure 3.10 Aerial view of Air Hitam survey line (Google Earth, 2018).

44

Figure 3.11 Soil laboratory test equipment.

45

Figure 3.12

PSD analysis equipment.

47

Figure 3.13

The position of current electrodes (C1 and C2) and

potential electrodes (P1 and P2) (Loke, 2001).

52

Figure 3.14 Electrode configurations of 2D resistivity method (Loke,

2001).

53

Figure 3.15 Wenner-Schlumberger electrode arrangement. The ‘n’

factor is the dipole separation factor (Loke, 2001).

53

Figure 3.16 2D resistivity and IP methods equipment.

56

Figure 3.17 2D resistivity method setup on field.

57

Figure 4.1 Rainfall distribution at Sungai Batu. (a) Seven days

cumulative rainfall (b) Rainfall distribution on 6th day and

7th day.

68

Figure 4.2 Inversion model of (a) 2D resistivity and (b) induced

polarization method at Sungai Batu study area.

71

Figure 4.3 2D resistivity inversion model for August 2016 at Sungai

Batu.

72

Figure 4.4 2D resistivity inversion model for September 2016 at

Sungai Batu.

72

Figure 4.5 2D resistivity inversion model for November 2016 at

Sungai Batu.

73

Figure 4.6 2D resistivity inversion model for December 2016 at

Sungai Batu.

74

Figure 4.7 2D resistivity inversion model for January 2017 at Sungai

Batu.

74

Figure 4.8 Sungai Batu weathered granite zone changes throughout the

monitoring period.

76

xii

Figure 4.9 Monitoring resistivity values at distance 30 – 80 m at

Sungai Batu study area.

77

Figure 4.10 Sungai Batu weak zone changes during the monitoring

period.

78



79

Figure 4.12 Sungai Batu granitic bedrock changes throughout the

monitoring period.

80



81

Figure 4.14 Rainfall distribution at Bukit Relau. (a) Seven days

cumulative rainfall (b) Rainfall distribution on 6th day and

7th day.

85


polarization method at Bukit Relau study area.

88

Figure 4.16 2D resistivity inversion model for September 2016 at Bukit

Relau.

89

Figure 4.17 2D resistivity inversion model for November 2016 at Bukit

Relau.

89

Figure 4.18 2D resistivity inversion model for December 2016 at Bukit

Relau.

90

Figure 4.19 2D resistivity inversion model for January 2017 at Bukit

Relau.

91

Figure 4.20 Bukit Relau fracture changes throughout the monitoring

period.

92

Figure 4.21 Monitoring resistivity values of distance 50 – 80 m at Bukit

Relau study area.

93


polarization method at Air Hitam study area.

96

Figure 4.23 Empirical correlation between porosity and moisture

content at Sg Batu and Bukit Relau study area.

97

Figure 4.24 Empirical correlation between resistivity and soil

laboratory tests.

98

xiii

Figure 4.25 Empirical correlation between chargeability and soil

laboratory tests.

99

Figure 4.26 Empirical correlation of resistivity and cumulative rainfall

of seven days including the survey day.

100

Figure 4.27 Empirical correlation of chargeability and cumulative

rainfall of seven days including the survey day.

100

xiv

LIST OF ABBREVIATIONS

2D Two dimension

3D Three dimension

4D Four dimension

ALERT Automated time – lapse electrical resistivity

tomography

ASTM American Society for Testing and Materials

BS British Standard

C1/C2 Current electrode

ERT Electrical resistivity tomography

FOS Factor of safety

GIS Geographic information system

IP Induced polarization

NPP North Penang Pluton

NW Northwest

P – wave Primary wave

P1/P2 Potential electrode

PSD Particle size distribution

RES2DINV Resistivity two-dimension inversion

xv

RMS Root mean square

S – wave Secondary wave

SE Southeast

SK Sekolah Kebangsaan

SMK Sekolah Menengah Kebangsaan

SP Self – potential

SPP South Penang Pluton

SW Southwest

TLERT Time lapse electrical resistivity tomography

USCS Unified Soil Classification System

xvi

LIST OF SYMBOLS

Vm measured voltage

Vr residual voltage

ρa apparent resistivity

∆t length of the time window of integration.

∇ gradient operator

△V Potential difference

Cc Coefficient of curvature

Cu Coefficient of uniformity

d diameter

D Effective size

E electric field intensity

e void ratio

I current

k geometric factor

M chargeability

ƞ Porosity

r distance of point in the medium

R Coefficient of correlation

xvii

R resistance

R2 Coefficient of determination

V electric potential

w Moisture content

ρ resistivity

ρbulk Bulk density

σ conductivity

ϕ phi unit

xviii

PENILAIAN KEGAGALAN CERUN DI PULAU PINANG MENGGUNAKAN

KAEDAH GEOELEKTRIK

ABSTRAK

Kegagalan cerun di kawasan berbukit dan kawasan cerun tidak berkurang

setiap tahun. Hal ini boleh membahayakan penduduk setempat. Walau bagaimanapun,

cerun tidak hanya terhad di kawasan penempatan tetapi juga boleh dilihat di sepanjang

lebuhraya dan jalan persekutuan. Kajian ini bertujuan untuk menyiasat cerun di

kawasan berisiko tinggi di Pulau Pinang (Sungai Batu, Bukit Relau dan Air Hitam)

menggunakan kaedah keberintangan 2D dan polarisasi teraruh serta menganalisa dan

mengaitkan keputusan dari keadah keberintangan 2D, kaedah polarisasi teraruh, ujian

makmal dan taburan hujan untuk potensi kegagalan cerun. Kawasan – kawasan kajian

dipilih berdasarkan kepada kajian terdahulu dan kes – kes kegagalan cerun yang

dilaporkan berlaku di Pulau Pinang. Berdasarkan kepada keputusan, kebarangkalian

untuk kegagalan cerun di Sungai Batu adalah disebabkan oleh kewujudan zon lemah

dengan nilai keberintangan rendah 0 – 400 Ωm and nilai kebolehcasan rendah < 3 ms

manakala Bukit Relau pula dijangka mengalami kegagalan cerun disebabkan oleh zon

luluhawa dengan nilai keberintangan pertengahan 750 – 3000 Ωm dan nilai

kebolehcasan pertengahan 3 – 10 ms. Kewujudan rekahan juga menyumbang kepada

kegagalan cerun terjadi. Sungai Batu dan Bukit Relau terletak berhampiran dengan

sesar, sekaligus meningkatkan potensi kegagalan cerun disebabkan oleh ciri – ciri

batuan yang kekar dan ricih. Air Hitam dikenalpasti terdedah kepada kegagalan cerun

disebabkan oleh kewujudan zon lempung tepu berdasarkan nilai keberintangan yang

rendah (< 400 Ωm) tetapi nilai kebolehcasan yang tinggi (> 25 ms). Ujian makmal

tanah juga dilakukan untuk mengenalpasti kandungan air dan keliangan. Pengamatan

xix

kolerasi antara nilai keberintangan dan nilai kebolehcasan diperoleh dari kaedah

geofizik dengan kandungan air dan keliangan dari ujian makmal tanah adalah berkadar

songsang antara satu sama lain dengan kekuatan kolerasi kuat sehingga sangat kuat.

Analisis taburan saiz zarah berjaya mengenalpasti tanah di Sungai Batu dan Bukit

Relau adalah pasir berkelikir. Cerun – cerun dipantau selama beberapa bulan untuk

memerhati sebarang perubahan sub permukaan. Berdasarkan keputusan di Sungai

Batu, zon lemah dengan nilai keberintangan < 400 Ωm ditemui di tengah garis tinjauan

manakala zon luluhawa dengan nilai keberintangan 500 – 3000 Ωm dikenalpasti

sepanjang garis tinjauan. Kaedah keberintangan 2D mendapati nilai yang menurun, ini

menunjukkan zon – zon yang semakin longgar. Rekahan juga dikesan terbentuk

semasa tempoh pantauan di jarak 130 – 140 m. Di Bukit Relau, fenomena yang sama

dapat diperhati dimana nilai keberintangan menurun dengan masa. Rekahan yang

dikesan di jarak 60 m bertambah lebar berbanding di awal tempoh pantauan dan

sebuah lagi rekahan dikesan terbentuk di garis tinjauan. Pemantauan cerun

membuktikan sub permukaan mengalami perubahan dan ia dipengaruhi oleh pelbagai

faktor seperti hujan di sepanjang bulan. Kajian ini berjaya mengenalpasti potensi

kegagalan cerun di kawasan – kawasan kajian iaitu zon lemah dan kehadiran bahan

lempung di Sungai Batu, zon luluhawa granit di Bukit Relau dan zon lempung tepu di

Air Hitam. Faktor yang membawa kepada kebarangkalian kegagalan cerun di kawasan

– kawasan kajian ialah keadaan tanah yang telap, taburan hujan yang tinggi dan juga

kesan luluhawa.

xx

SLOPE FAILURE ASSESSMENT IN PENANG ISLAND USING

GEOELECTRICAL METHODS

ABSTRACT

Slope failures never cease to end in hilly and slope area every year. This is very

dangerous especially for those who lives within the proximity. However, slope is not

only found in residential area but also alongside highways and federal roads. This

research aims to investigate slope at the selected highly suspected area in Penang

Island (Sungai Batu, Bukit Relau and Air Hitam) using 2D resistivity and induced

polarization methods and to analyse and correlate the results from 2D resistivity

method, induced polarization method, laboratory tests and rainfall distribution for

slope failure potentials. The study areas were selected based on the previous studies

and cases of slope failure reported in Penang Island. Based on the results, Sungai Batu

has probability for slope failure due to weak zone with low resistivity value of 0 – 400

Ωm and low chargeability of < 3 ms while Bukit Relau was expected to have slope

failure due to the weathering zone with intermediate resistivity 750 – 3000 Ωm and

intermediate chargeability 3 – 10 ms. The existence of fractures also contributed to

trigger the slope failures. Sungai Batu and Bukit Relau are located near faults and thus,

intensified the slope failure potential due to highly jointed and sheared rocks

characteristic. Air Hitam was determined to be vulnerable to slope failure due to the

presence of clay saturated zone indicated by low resistivity value (< 400 Ωm) but high

chargeability value (>25 ms). Soil laboratory tests were also conducted to determine

the moisture content and porosity. The empirical correlation of resistivity and

chargeability values obtained from geophysical methods with moisture content and

porosity from soil laboratory tests indicated that they are inversely proportional to each

xxi

other with strength of correlation range from strong to very strong. PSD analysis

successfully determined that the soil at Sungai Batu and Bukit Relau are gravelly sand.

The slopes were also monitored for several months to observe any changes within the

subsurface. Based on the results at Sungai Batu, weak zone with resistivity of < 400

Ωm was found at the centre of survey line while weathered zone with resistivity 500 –

3000 Ωm was identified along the survey line. 2D resistivity method recorded

decreases in value therefore indicated that the zones were becoming loose. A fracture

was also detected formed during the monitoring period at distance 130 – 140 m. As

for Bukit Relau, the same phenomenon was observed where the resistivity value

decreased with time. Fracture found at distance 60 m is wider since the beginning of

monitoring period and another fracture was observed to develop at distance 140 m

along the survey line. Monitoring the slope proved that the subsurface experience

changes and can be affected by various factors such as rainfall over the months. This

research has successfully determined slope failure potentials in the study areas which

are the weak zone and presence of clayey material at Sungai Batu, weathered granite

zone at Bukit Relau and clay saturated zone at Air Hitam. Factors that leads to slope

failures possibilities in the study areas are the permeable soil condition, high rainfall

distribution and also weathering effect.

1

CHAPTER 1

INTRODUCTION

1.0 Preface

Reported from cases of landslides, rockfalls and other types of slope failures

never cease to end every year. In the worst case, some of the slope failure repair

methods such as retaining walls, rock bolts and geo grid had collapsed together with

the slope failures. This perpetual issue will require an amount of money to be taken

care of.

Slope is not an uncommon structure in Malaysia. It can be seen alongside the

highways, federal roads and also residential areas. About 5000 km of the trunk roads

in Malaysia involved many cut slopes and traversed on hilly and mountainous area.

Approximately about 75% of the roads are underlain by granitic formation while the

other 25% of the roads are underlain by meta – sediments formation such as the

mudstone, sandstone and siltstone. Numbers of slope failure occurrences happen on

the mountainous roads especially during the rainy season which leads to traffic

disruption, injuries and also loss of lives (Singh et al., 2008). Although it happens

frequently, an absolute prevention method has yet to be found. Table 1.1 shows the

major slope failure occurrences in Malaysia in from 2002 – 2017.

Table 1.1: Series of major slope failure occurrences in Malaysia and consequences in

terms of deaths from 2002 – 2017.

Date Location Coordinate

Deaths Sources Longitude Latitude

20 November

2002 Taman Hillview, Hulu Klang 101.761500 3.175442 8

2

Continuation of Table 1.1

29 November

2004 KM 59, Kuala Lipis – Merapoh 101.961883 4.487183 4

Dep

artm

ent

of

Pu

bli

c W

ork

s M

alay

sia

2 December

2004 Taman Bercham Utama, Ipoh 101.135061 4.645389 2

31 May 2006 Kg Pasir, Hulu Klang, Selangor 101.764778 3.206783 4

26 June 2006 KM 8.5, Pelabuhan Sepanggar,

Kota Kinabalu, Sabah 116.674689 6.095556 1

30 November

2008

Ulu Yam Perdana, Kuala

Selangor 101.674689 3.389919 2

6 December

2008

Taman Bukit Mewah, Bukit

Antarabangsa, Hulu Klang 101.769722 3.186111 5

16 January

2009 Bukit Kanada, Miri, Sarawak 113.996494 4.393056 2

29 January

2011 Sandakan, Sabah 118.127078 5.847014 2

21 May 2011 Rumah Anak Yatim At –

Taqwa, Hulu Langat 101.814444 3.138611 16

7 August 2011 Perkampungan Orang Asli Sg

Ruil 101.370464 4.487361 7

p18 February

2012 Kg Terusan, Lahad Datu, Sabah 118.359314 5.048847 2

3 July 2013 Ukay Perdana, Selangor - - 3

Kazmi

et al.

(2016)

16 July 2013 Kampung Mesilau, Kundasang - - 1 Utusan

30 December

2014 KM Jalan Brinchang – Tringkap - - 2

Astro

Awani 31 December

2014

Kampung Raja, Cameroon

Highland - - 1

22 October

2017 Tanjung Bungah, Penang - - 14 Reuters

Shallow slide is the most common type of slope failure in Malaysia where the

slide surface is usually less than 4 m depth and occurs during or immediately after

intense rainfall (Jawaid, 2000). Other types of slope failure found are deep-seated

slide, debris flow and geologically controlled failures such as wedge failures and rock

falls.

By definition from Frasheri (2012) slope is a dynamic system of geo-

environment phenomena that are related to the movement of soil and rock masses.

Some examples of slope failure including avalanche of the rocks, debris and soil flow,

collapse of the blocks and landslides. Slope failure phenomena can be defined as an

3

engineering and environmental issue. In order for a slope to fail, there is a process

related to deformation of the rock mass, accumulation of stress and mineralogical

changes involved. The body movement only occurs as the last stage and associated

directly with mechanical and geological condition of the rock mass. The slope become

unstable as the rock mass loss mechanical stability due to full compliance of the

unstable condition. The rock mass sensitivity to the external force also increased as it

become more unstable.

There are many factors that affected slope stability therefore causes slope

failure to occur. One of the factor is due to gravitational force which pulls everything

to the direction of Earth’s centre. Unstable rock or structure will allow the gravitational

force to act and end up as landslide or rockfall. Another major factor that influences

slope stability is water. Additional of water on the subsurface will results as the slope

to increase in weight therefore becomes unstable. Oversaturated soil with water will

cause the grain to lose contact to one another and can also change the angle of repose.

The nature of Earth material such as clayey type of soil and the presence of structure

such as fracture and weathered zone in the subsurface can also cause slope failure to

occur. On top of all, the most critical factor for slope failure to occur is triggering

events such as heavy rainfall and earthquake (Nelson, 2013).

1.1 Problem statements

Aside from the slope at the roadsides, residential areas and agricultures are

developed on the slope areas too. The limited flat land in Penang Island has urged

developers to develop buildings on the steep slopes (Teh, 2000). This irreversible

action has taken tolls on the hills ecosystems. As the developed slope areas spreads,

the effects could be more severe and worse. On top of that, illegal forest clearing and

4

illegal farming on slopes without any consideration of the subsurface condition could

accelerate soil erosion and slope failures. Table 1.2 shows the summary of slope

failures in Penang Island in 2008 to 2017. This indicates that slope failure in Penang

Island is a crucial matter.

Table 1.2: Summary of slope failures in Penang Island.

Date Location Remarks

4 October 2008 Persiaran Kelicap, Sungai Ara

Collapsed retaining wall

caused flooding and

mudslide to resident houses.

7 October 2008 Jalan Teluk Kumbar – Balik Pulau Landslide caused road

closure to traffic

6 April 2009 Pangsapuri Farlim Tower, Bandar

Baru Farlim, Air Hitam

Landslide, damage on

vehicles

10 June 2010 Pangsapuri Bandar Baru Air Hitam Landslide

24 June 2010 Batu Feringghi

Mudflow and mudslide from

nearby hillslope

development project

10 March 2011 Taman Terubong Jaya

Landslide, caused collapsed

of retaining wall and buried

two cars

September 2013 Penang Hills Landslide at KM 2.1, KM

2.5 and KM 4.0

29 October 2016 Air Hitam

Landslide near Kek Lok Si

Temple heading to Air

Hitam Dam

7 November 2016 Jalan Ujung Baru, Teluk Bahang Landslide forced road

closure to traffic

5 November 2017 Tanjung Bungah Three newly built luxury

houses destroyed.

In accordance to slope failure occurrence, Department of Public Work

Malaysia will act according to Malaysia Standard Code Practice MS 2038: 2006: Site

Investigation – Code of Practice which included the application of boring log, ground

water observation and Mackintosh probe test and Malaysia Standard Code Practice

MS 1056: 2005: Soil for Civil Engineering Purpose – Test Method which included

laboratory test of disturbed and undisturbed soil samples. Slope failure is a perpetual

issue in Malaysia, any method that can be in assistance to solve such issue should come

to the front. In this research, the ability of geophysical methods in dealing with

engineering and environmental issue are tested to determine the slope failure potential.

5

The common geophysical methods apply for slope failure studies are 2D

resistivity method and seismic refraction method (Popescu et al., 2014; Giocoli et al.,

2015; Muztaza et al., 2017; Hazreek et al., 2017). In this research, the application of

2D resistivity method is paired with induced polarization method as this combination

in slope failure studies is yet to be well-known (Marescot et al., 2008).

1.2 Research objectives

The objectives of conducting this research is listed as follows:

i. To investigate slope at the selected highly suspected area in Penang

Island (Sungai Batu, Bukit Relau and Air Hitam) using 2D resistivity

and induced polarization methods.

ii. To analyse and correlate the results from 2D resistivity method,

induced polarization method, laboratory tests and rainfall distribution

for slope failure potentials.

1.3 Scope of research

This research was conducted at Penang Island which is generally made up of

granitic rock. The three study areas were chosen based on the landslide distribution in

Penang Island obtained from previous studies and also cases of landslide reported.

The geophysical methods used in this study were 2D resistivity method and

induced polarization (IP) method by using ABEM SAS4000 Terrameter and ABEM

ES 10-64C electrode selector. The resistivity inversion models were produced by using

RES2DINV software by Geotomo. Each model inversion was studied and observed so

that any slope failure possibilities can be detected.

6

Aside from geophysical methods, the soil laboratory tests were also conducted

in order to measure the soil porosity, moisture content and particle size distribution.

Rainfall distribution were also factored, in order to identify the potential of the slope

failures.

1.4 Significance of study

People tend to use the ground methods, geotechnical methods and remote

sensing over the geophysical methods (Talib, 2003; Ahmad et al., 2006; Khan et al.,

2010; Lateh et al., 2011; Department of Public Works Malaysia, 2018). There are

many reasons that geophysical method should be chosen when dealing with

engineering and environmental issues. The geophysical methods are non – destructive,

low cost consumption and have wider area coverage. Geophysical methods also have

the potential in dealing with engineering and environmental problem such as slope

failures.

This research aims to apply geophysical methods in dealing with engineering

and environmental problems Thus, this research is carried out in order to convince the

potential lies within geophysical methods in dealing with engineering and

environmental problems.

By applying geophysical methods in slope study, the expenditure can be

reduced and remedial works will be done in more efficient ways and not blindfolded.

The significant of this study is to understand the mechanism of slope failures at highly

potential areas in Penang Island as most of the study conducted before were site

specific. Another significant of this study is the application of induced polarization

method which rarely used for slope study. This aims to detect saturated zone and high

moisture area in the subsurface and its origin.

7

1.5 Thesis outlines

This thesis contained five chapters as follows:

Chapter 1 is the introduction of the research where the preface of study,

problem statements, research objectives, scope of study, significance of study, and

thesis outline are explained.

Chapter 2 is the literature review. The background of the slope is elaborated,

theory of 2D resistivity and IP method is explained and the previous study that related

to this research are discussed.

Chapter 3 is the materials and methods chapter. The details about the study

area including the geological of the area and the survey lines orientation are shown.

This chapter also explained the equipment used and the procedures during data

acquisition. Aside from that, procedure that related to soil laboratory tests are

explained as it is part of the research element.

The results are discussed in Chapter 4. This includes the soil laboratory tests,

particle size distribution results and geophysical results. The results are correlated to

the rainfall distribution. Empirical correlation between laboratory test and geophysical

results are also shown in this chapter.

Chapter 5 is the conclusion and recommendations. The research findings are

concluded and discussed. Some recommendations for future works are also suggested

in this chapter.

8

CHAPTER 2

LITERATURE REVIEW

2.0 Introduction

For engineering and environmental purposes, people tend to choose the

airborne and satellite methods such as remote sensing and Geographic Information

System (GIS) or the direct ground-based techniques such as piezometer and laboratory

tests over geophysical method which are also capable in solving the engineering and

environmental problems (Talib, 2003; Ahmad et al., 2006; Khan et al, 2010; Lateh et.

al, 2011). Fortunately, people had grown interest in geophysical method in order to

investigate engineering and environmental problems such as slope stability in the last

several decades (Jongmans and Garambois, 2007; Chambers et al., 2011). Each

geophysical method has their own specialities on specific targets and environments

which will produce good results besides time and cost effective. Several studies on

slope failures and the application of geophysical methods on slope stability are

included in this chapter.

2.1 Previous studies

A landslide on argillaceous material occurred in Western Cameroon around

Kekem area. Geophysical and geotechnical surveys were carried out by Epada et al.

(2012) with purpose to understand the triggering processes and mechanism of the

landslide and to assess the slope stability. Resistivity survey had been conducted by

using electrode configuration of Schlumberger array in order to monitor the behaviour

of the landslide in term of resistivity. A zone of low resistivity value was identified as

clayey sand-filled aquifer in the subsurface of the landslide. This aquifer was suspected

9

to be the triggering factor of the landslide. Based on the geotechnical sounding result,

the aquifer had a thickness of 7.0 m whereas the depth of the landslide crest level to

the failure surface reached 3.0 m and 20.6 m. Next, laboratory tests were carried out

to evaluate the cohesion of the soil and the angle of internal friction. The tests result

shows that the soil has low consistency which is almost doughy. As for stability

analysis, the mean value of the factor of safety (FOS) is 1.4 which was lower that the

slope stability coefficient which is 1.5. Therefore, the slope was unstable and likely to

reactivate at any moment.

A landslide was studied at Buzad village, Timis County, Romania by using

electrical resistivity tomography (ERT). The landslide occurred in 2006 was

reactivation of an old landslide. Popescu et al. (2014) conducted three resistivity

survey over the main body of the landslide by using electrode configuration of Wenner

array in 2007, 2012 and 2014 (Figure 2.1). The length of the profile was 90 m in 2014

but 100 m in 2007 and 2012 and achieved depth of about 15.0 m along all longitudinal

profiles. The results showed that a section with resistivity value of 0-35 Ωm at the

upper half while the middle part ranged from 55 – 100 Ωm. The bottom part recorded

as shallow high resistivity zone. This suggest that high water content from the middle

part of the profile may be the triggering factor of the reactivation. The hypothesis was

confirmed from an in-situ observation of the ground water table appeared to the

surface. Whereas the high resistivity value of the bottom part is compact material

(sands) embedded in clayey matrix of the old landslide body. Resistivity method

allows one to identify high water content area that cause the reactivation and thus

reconstruction of the landslide body including the body materials in movement and its

volume. Aspect noted in all resistivity profiles, the new and old landslide were located

in a zone of obviously changing resistivity values.

10

Figure 2.1: ERT profiles through Buzad landslide in (a) 2007 (b) 2012 (c) 2014

(Popescu et al., 2014).

Glacial erosion had produced ‘crag and tail’ structure at the Edinburgh Castle

which is one of the most important heritage site in Scotland. The crag consists of

columnar jointed basal that formed a gentle slope protecting the tail of sediment from

glacial erosion. An apparent instability was observed in the southern side of the ‘tail’

between the Edinburgh Castle Esplanade and Johnston Terrace in 1997 which later

initiated the geotechnical and geological investigation on the particular slope including

(a)

(b)

(c)

11

electrical resistivity survey. In order to determine the subsurface structures and

variations in pore fluid distribution within the slope, Donnelly et al. (2005) conducted

six resistivity survey lines with length of 24 m with electrode configuration of Wenner

array. Line 1 to Line 5 were positioned down the slope perpendicular with the southern

boundary wall of Esplanade whereas Line 6 was positioned across the slope parallel

to the Esplanade wall. A shallow slope failure was confirmed based on the result

however the initial date of failure is not known but likely since at least 1950’s. The

resistivity profiles indicated that the instability was due to zones of low resistivity and

high saturation. The backscarp also seems to be associated with relatively thin clay

layer which apparently not a slip plane but however may cause the local groundwater

to flow in as a highly permeable fill material. Similar condition was found to the east

and west of the uppermost featured of the landslide lateral continuation.

Giocoli et al. (2015) performed a joint analysis of geophysical surveys, aerial

photos interpretation, morphotectonic investigation, geological field survey and

borehole data to investigate an area in the Montemurro territory in southeastern sector

of High Agri Valley (Basilicata Region, southern Italy). Generally, the area consists

of steep slopes, encased streams within narrow and deep land incision and abrupt

acclivity change due to tectonic structures or lithological variation. This area has been

affected by the hydrogeological instability which included active and inactive

landslides. Surficial investigation identified the existing of northwest (NW) –

southeast (SE) trending and southwest (SW) facing scraps between villages of Pergola

to the north and Voggioni to the south. Geophysical surveys were conducted to unravel

the surface expression of the strand. Three resistivity survey lines were carried out

across the NW-SE scarp by using Wenner-Schlumberger electrode configuration with

20 m spacing. Survey line ERT1 and ERT3 achieved depth of penetration of 150 m

12

with spread of 940 m length whereas survey line ERT2 achieved the same depth with

spread of 1100 m length (Figure 2.2). In the upper of line ERT2, a body associated

with the Verdesca landslide was identified between 430 m and 1100 m based on the

resistivity values (25 – 70 Ωm). The estimated thickness was derived from borehole

data GB1 – GB3 and resistivity contrast from ERT2, varies from less than 10 m and

up to 30 m. Another geophysical method done was a high-resolution seismometer with

24-bit dynamic was aimed at the very low amplitude range. Thirty-five ambient noise

measurements with each duration of 12-15 minutes with additional measurements

overlap with all resistivity profile lines were taken. The results successfully showed

the fundamental frequency was related to the depth variation of the seismic impedance

differ between the low resistive Quartenary continental deposits (QD) and Albidona

Formation (AF) and high resistive Gorgoglione Flysch (GF). These joint analyses

succeed in achieving the objectives of the study which is to image the shallow

geological and structural setting as well as to verify the nature of the NW-SE scarps

and thus interpreting it as surface expression of the Montemurro Fault.

13

Figure 2.2: Resistivity profiles for ERT1 – ERT3 (Giocoli et al., 2015).

A study of landslide was conducted by Drahor et al. (2006) at district of Aydin,

Turkey in 2003 on a slope next to a newly built school building. The slope was unstable

due to an excavation work and heavy rainfall. Three resistivity survey lines were

conducted over the landslide using Wenner electrode array resulting on the geometry

and characteristic of the landslide. Eight boreholes were also carried out on the

landslide. Both methods showed a fault presence at the area with addition of a surface

rupture indicated by the resistivity profiles (Figure 2.3).

14

Figure 2.3: 3D fence diagram for resistivity section in Aydin (Drahor et al., 2006).

Resistivity and self-potential methods were applied to study the hydraulics of

landslide process as these methods are capable to provide spatial and volumetric

information as well as sensitive toward hydraulic changes in the subsurface. A system

called Automated Time-lapse Electrical Resistivity Tomography (ALERT) was

applied on an active landslide near Malton, North Yorkshire, United Kingdom in order

to develop a 4D landslide monitoring system that capable to characterise the

subsurface structure of landslide and detecting the hydraulic precursor to movement.

According to Chambers et al. (2009), the time-lapse resistivity imaging was able to

show the changes related to the seasonal temperature variation, moisture content and

ground movement within the landslide. The 4D resistivity imaging was an effective

method to investigate the hydraulic of a landslide whilst taking the influence of

temperature and electrode displacement into consideration.

One of study conducted in Malaysia is to investigate the influence of natural

slope geomorphology on active cut slope failure near Gunung Pass, Simpang Pulai-

Lojing Highway by Shuib et al. (2012). Previous studies have found out that there was

close correlation between the relict landslide scars on the natural slope and the

15

presence of cut slope instabilities. Hence resistivity and seismic refraction surveys

were conducted across the relict landslide scars. Based on the results, the steep relict

scarps were connected at the subsurface by a circular slip surfaces which is represent

by steep weak zones. The head scarp zone was represented by the topmost slip surfaces

with tension cracks suggestive of extensional strains while the toe region was

represented by the reversed slip surface suggestive of compressional strains. The

failure was due to the head and the upper main body zone sliding roughly orthogonal

to the foliation while the middle and toe zone is sliding down and out of the foliation.

From the surface and subsurface observation, the natural slope was inferred to be

creeping which later induces shallow planar and circular failures at the cut slope.

Based on a study conducted by Chigira et al. (2011), landslides in weathered

granitic rocks are greatly influence by the type of weathering and it is site specific. The

mechanism of failure in deep weathering profiles in Malaysia are highly controlled by

the nature of the weathered material and its mass structure. In Japan, the landslides

occurred due to moderately weathered decomposed granite. The rock loosened rapidly

as it was exposed to the ground surface and formed loosened layers to slide. The fact

of weathered granite is prone to slide was known for many years. However, the

landslide mechanism was not yet fully understood due to the weathering profile was

site specific and differ among climates. More than 80 landslides occurred along the

mountainous main highway to Genting Sempah, Pahang. Most of the landslides

occurred in a few hours in June 1995 as there was a continuous rain for more than 72

hours. Majority of the landslide can be categorized as small to medium size with

sliding materials less than 500 m3. Types of slope failure included shallow and deep

sliding, rock block sliding and debris flow. This is a region that formed by granite

batholiths and the granite has undergone intensive tropical weathering. This resulting

16

in forming weathered profiles with various characteristic and thickness. Most of the

landslides are related to the weathered materials. The intense and heavy rain saturate

the residual soil thus, triggering two major landslides upstream of a tributary of Sungai

Gombak.

A study conducted by Komoo (1997) at Bukit Antarabangsa concluded that

the weathered granitic material may contributed to landslide as it was porous, easily

crumble and inherited the plane of weakness from the parent rock. Bukit Antarabangsa

has marked in the history of Malaysia landslide disasters as there were six major

landslides occurred since 1993. The hill was underlain by granite and extensive

weathering has transformed the granite into residual soil (grade VI) and completely

weathered material (grade V). The weathered material was sandy with average

thickness profile of 30 m. The subsurface loose its consistency with increasing amount

of water. This condition triggered a collapsed of twelve storey Highland Tower

condominium in 1993.

Another study conducted by Komoo and Lim (2003) at the same Bukit

Antarabangsa with 100 m distance south from the previous landslide. In 2002, a

bungalow was destroyed due to landslide killing eight people. The landslide

mechanism was rather complex but it is due to continuous heavy rain that triggered

the sliding. That was not all, other factors may also contribute the triggering of the

landslide. Factors such as weathered granitic materials that prone to failure, geological

lineament that facilitating the sliding. An old landslide landform found nearby also

aided the accumulation of groundwater that resulting in the landslide in 2002.

Muztaza et al. (2017) has conducted a slope failure evaluation and landslide

investigation by using 2D resistivity method in Selangor. Nine survey lines were

17

carried out at Site A and another six survey lines at Site B with minimum electrode

spacing of 5 m were performed using Pole – Dipole electrode array. Alluvium or

highly weathered zone with resistivity value of 100 – 1000 Ωm found at depth > 30 m,

saturated area with resistivity value of 1 – 100 Ωm and boulders with resistivity value

of 1200 – 7000 Ωm. The granitic bedrock was identified with resistivity value of >

7000 Ωm (Table 2.1). From this study, the triggering factors for slope failure was due

to the saturated zones, highly weathered zone, highly contain of sand, and boulders.

Table 2.1: Summary of resistivity value with interpretation at the study areas in

Selangor (Muztaza et al., 2017).

Resistivity value (Ωm) Interpretation

1 – 100 Saturated zone

100 – 1000 Alluvium or highly weathered zone

1200 – 7000 Boulder

> 7000 Granitic bedrock

A slope failure in Kenyir Lake was evaluated by Hazreek et al. (2017) using

resistivity method with a reference to a borehole data. Two resistivity survey line was

carried out using electrode array of Schlumberger array. The main factor that triggered

the slope failure is the combination of heavy rainfall and the presence weakness zone.

The results also successfully detected fault and rock discontinuities which associated

by low resistivity value (Figure 2.4). By applying resistivity method, the shape and

depth of subsurface landslide that caused ground damage can be determined easier as

the resistivity method mapped the subsurface profile based on the resistivity values.

18

Figure 2.4: Resistivity profiles according to the location (a) top of landslide (b) toe of

landslide (Hazreek et al., 2017).

Yaccup and Tawnie (2015) had conducted a comparison between resistivity

imaging technique and borehole data on determining depth of granite body as the

active quarry site in order to define the accuracy of the resistivity data with borehole

information. Eight resistivity profiles were built near to several boreholes with

maximum distances of less than 50 m. The result shows that in the tropical region such

as Malaysia (Table 2.2), the resistivity value of low weathered granite to fresh granite

is more than 5000 Ωm, medium weathered between 1000 – 5000 Ωm and highly

weathered granite is lower than 1000 Ωm. Resistivity and borehole recorded accuracy

more than 80% with another 20% error due to changing of resistivity due to existence

of water, fractured zone and the distance between the survey line and borehole.

Table 2.2: Resistivity range of weathered granite and fresh granite in Malaysia

(Yaccup and Tawnie, 2015).

Resistivity value (Ωm) Interpretation

< 1000 Highly weathered granite

1000 – 5000 Medium weathered granite

> 5000 Low weathered – fresh granite

(a)

(b)

19

Jinmin et al, (2013) applied 2D resistivity method for subsurface study at

Bukit Bunuh. The aim was to identify the bedrock structure, fracture of shallow

subsurface and identify the alluvium pattern. Due to large study area, the borehole data

is not adequate to measure the subsurface condition. Therefore, the borehole data were

used as preliminary information to refine 2D resistivity results. A survey line of 2.065

km was done by applying Pole – Dipole array with 5 m minimum electrode spacing.

The results show two major zones in the study area; resistivity value of 10 – 800 Ωm

interpreted as alluvium and resistivity value of more than 3000 Ωm as boulder and

granitic bedrock.

A study on hill-slope movement was conducted by Khan et al. (2010) to

monitor a land movement during rainy seasons in 2008. Two landslides occurred

between 23 and 26 km stretched through Gunung Pass, Cameron Highland in April

and December 2007 respectively. Based on deduction, the hilly terrain was triggered

by the rainfall events. Moreover, the slope at Gunung Pass may experience some

movement within the subsurface from the past rainfall events before the failure

occurred in 2007. Therefore, a theodolite study was conducted on the slow hill-slope

movement to monitor the land movement during rainy seasons of 2008. The results

successfully identified a close correlation between the hill-slope displacements and the

rainfall amount during the study period. The slow hill-slope movement occurred after

a heavy rainfall in 2008. Result from Prism III theodolite showed that the down slope

movement was more than 4 m, southward movement of 1.94 m and westward

20

movement of more than 5 m (Figure 2.5). As conclusion, the area was unstable and

prone to slide.

Figure 2.5: a) Average rainfall at Cameron Highland from March 2008 to Dec 2008 b)

Observed total displacement at Prisms I-VI (Khan et al., 2010).

Xu et al. (2016) investigate a landslide in southwestern China by using time –

lapse electrical resistivity tomography (TLERT) in November 2013 and August 2014.

The study aims to investigate the subsurface hydrogeological environmental and

evolution of landslides based on the electrical structure and geometry of sliding

surface. The landslide mechanism is inferred based on the spatiotemporal

characteristic of the surface water infiltration and groundwater flow within the

(a)

(b)

21

landslide body. The resistivity inversions accurately defined the interface between the

Quarternary sediments and bedrock when combined with borehole data. The results

show that the surface water penetrates via the fracture zone and fissures into the

slipping body and drained as fissure water in the fractured bedrock. This eventually

caused the weathered layer to soften and erode. This finding indicated that the TLERT

monitoring able to provide preliminary information on critical sliding and can be used

for landslide stability analysis and prediction.

A study conducted by Rahardjo et al. (2001) in Nanyang Technology

University Campus successfully indicated that the landslides were initiated by the

rainfall infiltration. The daily rainfall and the cumulative rainfall both act as the

triggering factor for slope failure. A five days cumulative rainfall with more than 60

mm combined with daily rainfall exceeded than 90 mm was sufficed to initiated a

landslide.

A water – seepage coupling model and stability analysis was developed to

study the effect water on the soil – slope stability by Liu and Li (2015). In order to

analyse the effect of water seepage which came from rainwater and water level

fluctuation, safety factor of slope by rainfall and water level fluctuation was simulated.

The simulated result indication that both rainfall and water level fluctuation decreased

slope stability according to the safety factor of the slope (Figure 2.6). The reason for

this phenomenon is increased in total soil weight and slight improvement of pore

pressure in slip surface. Therefore, slope stability is highly affected by rainfall and

water level fluctuation.

22

Figure 2.6: Safety factor caused by reservoir water, rainfall and combination of rainfall

and reservoir water fluctuation (Liu and Li, 2015).

Lateh et al. (2011) monitored the slope movement before the slope failure

occurs by using inclinometer, piezometer and rain gauge at 3.9 km Jalan Tun Sardon,

Penang. The soil was detected move on 30th July 2010 and 4th November 2010 due to

the intense rainfall before the occurrence. Therefore, the soil movement and rainfall

intensity can be related in affecting slope failure occurrence.

A slope investigation was carried out at Fraser’s Hill due to a debris flow was

found at 9 km of Fraser Hill road by Ghazali et al. (2013). Using resistivity method,

three survey lines parallel to the slope and two survey lines perpendicular to the slope

were applied at the reported area. Results showed existence of granite body at the

central part of the east line and nearest to the ground surface. The presence of

resistivity less than 600 Ωm within the granite body is interpreted as highly fractured

and water conducting zones. The same zone was detected at east – west and north

south lines. Therefore, the fracture granite is virtually floating above the water

saturated zone thus, considered as unstable.

F

ac

tor

of

sa

fety

23

Ng et al. (2015) has utilized resistivity method to investigate a slope failure at

Precinct 9, Putrajaya with aided by borehole sampling data. Results showed two zones

were encountered, high water content zone with resistivity value of 10 – 300 Ωm and

dry zone with resistivity value of 300 – 100 kΩm. Borehole sampling recorded

subsurface condition was range from fresh to highly weathered granite. Rock quality

designation (RQD) indicated value of 0 – 100 % which show the presence of rock

fracture that cause seepage flow within the subsurface. Therefore, low resistivity zone

face risk for slope failure which demand remedial measures.

Ramadhan et al. (2015) has conducted resistivity method by using electrode

configuration of Wenner – Schlumberger at Bendanduwur Gajahmungkur Semarang

to identify landslide slip surface. The results of resistivity method reached to depth of

14 m and it shows that the lithology of the location consist of soil, clay, sandstone and

breccia. The contact between clay and sandstone found at depth 1.25 to 6.76 m found

in the survey lines is interpreted as the slip surface of landslide due to the contact

indicated disconnection between the layers also known as weak zone. Landslide can

be expected in this location due to the presence of clay, which is impermeable for water

to pass through.

A study by Ahmad et al. (2006) at Paya Terubong on a landslide that occurred

in 1998. The average thickness of highly to completely weathered granitic soil is found

to be approximately 30 m. The sampled residual soil consists of gravel (14%), sand

(55%), silt (18%) and clay (13%) which can also represent as course-grained residual

soil which is also known to be susceptible to landslide.

Hashim et al. (2015) used shear box test in order to identify slope failure in

Penang Island. This study aims to determine the soil shear strength under saturated

24

shear box test for samples taken from slope failure locations. The locations were

selected from slope failure tragedy sites along Teluk Bahang – Balik Pulau road. The

summary of shear box test and particle size distribution test (BS 5930: 1990) is as in

Table 2.3 and Table 2.4 respectively. The results indicated that the slope failures were

found mostly in gravelly silt soil and the range of cohesion recorded was largest

compare to others soil.

Table 2.3: Saturated peak shear box and particle size distribution tests, BS 5930: 1990


Soil types Range of cohesion Range of angle of shearing resistance

Silt 0.1 – 33.4 22.9 – 47.1

Very silty sand 0.6 – 8.3 30.7 – 62.4

Sandy silt 0.0 – 27 18.6 – 52.9

Very silty gravel 0..8 – 37.5 24.5 – 57.2

Gravelly silt 0.0 – 40.8 18.1 – 65.8

Table 2.4: Result of soil classification and particle size distribution test, BS 5930: 1990


Soil

sample Slope condition

Percentage (%) Soil classification

Gravel Sand Silt Clay

A Unfailed 64.88 18.08 17.04 0.00 Very silty gravel

B Failure mass 37.92 20.67 41.25 0.16 Gravelly silt

C Failure scar 36.26 30.16 33.53 0.05 Gravelly silt

D Failure mass 36.88 24.21 38.67 0.24 Gravelly silt

E Failure mass 26.19 36.88 36.85 0.08 Gravelly silt

F unfailed 76.93 7.45 15.50 0.12 Very silty gravel

A study conducted by Bery (2016) to identify the empirical correlation of soil

properties of shear strength, moisture content, void ratio, porosity, saturation degree

and Atterberg’s limit with electrical resistivity values from resistivity tomography

models. Eleven undisturbed clayey sand soil samples were collected at different

distances, depths and both infield and laboratory condition. Based on the results, the

electrical resistivity values were highly influence by the soil properties. A good

estimation of soil mechanics properties can be obtained from the results of electrical

resistivity tomography model.

slope failure assessment in penang island ...eprints.usm.my/44257/1/muhamad iqbal mubarak...

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